This sounds like the well documented trans "lurch" problem. Many people have experienced this and have described it in the same way that you have. If that's the case, then what you are hearing is the trans down shifting from 2nd to 1st and you get a bang from the rear as 1st engages. it's not always the same severity.

It's not only on the BMW's.
Jaguar S-Type ZF Gearbox Lurch fault

The symptoms can be reduced by performing the "30 second" trans memory reset. This will reset the shift patterns back to their defaults. However, the symptoms will come back.

The transmission program update may be available for your model. As far as I've learned, this does not always fix the issue.

My friend who is a transmission engineer recommended replacing the trans filter and fluid, and also cleaning the trans valve body. The filter is recommended by some BMW experts to be replaced every so many miles:
BMW Transmission

The valve body can be removed and opened carefully. Use a baking sheet on your work bench to contain parts and fluid. There are many little springs in there that need make it back in when you close it up.

For the M62 engine, the valve body is shown here:
RealOEM.com BMW E53 X5 4.4i A5S440Z CONTROL VALVE ASSY